The sticking of drilling or production equipment in an oil or gas well bore requires that an axial blow be delivered to unstick the equipment. Downhole tools known as “jars” have been used in such situations. One type of jar is a “drilling jar.” Another type of jar is a “wireline jar.” In the case of a wireline jar, a series of impact blows is delivered to the stuck equipment by manipulation of the wireline. Wireline jars typically have an inner mandrel and an outer housing telescopically coupled together for relative axial, sliding movement. The mandrel carries a hammer and the housing carries an anvil. By directing the hammer to impact the anvil at high velocity, a substantial jarring force may be imparted to the stuck equipment, which is often sufficient to jar the stuck equipment free. A wireline jar is shown and described in U.S. Pat. No. 6,481,495, which is hereby incorporated by reference in its entirety.
There are various types of jars: mechanical, hydraulic, and mechanical-hydraulic. Each type is cocked and subsequently fired to deliver the impact blow. A trigger mechanism initiates firing of the jar by retarding relative motion of the hammer and anvil until an axial strain has been applied to the drill string pipe sufficient to actuate the trigger mechanism. Typically, an axial tensile force applied at the surface pulls on the wireline and thus the mandrel. The trigger mechanism resists the tensile force and causes potential energy to be stored. When the jar trigger mechanism fires, the stored energy is converted to kinetic energy and the hammer hits the anvil.
The trigger mechanism in a mechanical jar includes a spring to resist movement of the mandrel relative to the housing. The spring has a constant response such that a certain amount of applied force applied to the mandrel is required to compress the spring a given amount. A collet is coupled to the mandrel and moves with the mandrel as the spring is compressed under the applied force. The collet and a trigger sleeve keeps the mandrel engaged against the resisting force of the spring. When the applied force on the mandrel exceeds a predetermined amount (i.e., the triggering load), the spring will have been sufficiently compressed for the mandrel to have moved a sufficient distance relative to the trigger sleeve for the collet to release the mandrel, whereupon the jar “fires.”
The trigger mechanism in a hydraulic jar includes a piston to pressurize fluid in a chamber to resist movement of the mandrel relative to the housing. The pressurized fluid bleeds off at a predetermined rate. Eventually a pressure is reached at which a chamber seal is opened, and the compressed fluid is allowed to rush out, firing the jar by freeing the mandrel to move rapidly in an axial direction. In a hydraulic jar, the trigger mechanism is not over-pull force dependent; it will trigger at any load that is pulled following a time delay. Advantageously, a hydraulic jar as disclosed in U.S. Pat. No. 6,290,004 includes a mechanical lock preset to trigger at a load greater than the weight hanging below the jar and a hydraulic time delay that allows the jar to be actuated at loads higher than the lock setting without the need to open a chamber seal.